The polycylic skeleton of derivatives of the formula I may be compared with the one of vincamine wherein the cycle D is missing. This skeleton is identical to the one of canthin-6-ones which form a class of natural alcaloids (see for example L. A. Mitsher et al., Heterocyles, 3, 7, 1975).
We may thus described the compounds of the invention as derivatives of canthinones, canthinols or canthenes when, respectively, R5 and R6 represent in formula I an oxygen atom, R5 is a hydroxyl group and R6 a hydrogen atom or R6 represent a hydrogen atom and R4 and R5, together, is a additional C--C bond. The 1,2,3,3a,4,5-canthin-6-one skeleton corresponds to the 2,3,3a,4,5,6-hexahydro-6-oxo-1H-indolo(3,2,1-de)(1,5)naphtyridine.
Belgian Pat. No. 853.435 discloses the preparation of a large number of differently substituted indolonaphthyridines. Among these, we may more particularly mention the 4-carboalkoxy-hexahydro-indolonaphthyridines.
Belgian Pat. No. 870.887 (U.S. Pat. No. 4,200,638) describes compounds of the formula I wherein R1=R2=H, R5 is a carbomethoxy group and R4 and R6 are together an additional carbon-carbon bond. They have been named chano-desethylapovincamines or "small apovincamines". Numerous similar derivatives are described in Belgian Pat. No. 884.145.
Belgian Pat. No. 857.816 (U.S. Pat. No. 4,218,453) describes and claims compounds of the formula I wherein R1 and R2 represent hydrogen atoms and R5 and R6 represent together an oxygen atom. These are also called hexahydrocanthin-6-ones derivatives. Belgian Pat. Nos. 816.759 and 882.024 describes the preparation of seco-D-homo-E-vincamones (also named chano-D-homovincamones) substituted at a position corresponding to the position 4 of formula I.